The measurement of the physical properties of a material with nano- or sub-nanometer dimensions (hereon referred to as “point contacts”) is non-trivial. With smaller dimensions such measurements become progressively more difficult due to several complicating factors including: (a) weak signal from small samples; (b) inadequate signal-to-noise ratio; (c) parasitic noise (electrical and mechanical); (c) uncertainty in sample composition; (d) large fluctuations in signal even with small variations in temperature, pressure, humidity, contamination, etc.; (e) sample stability; (f) lack of universally acceptable standards for the measured physical property; and (g) traceability, etc. Moreover, in many instances, a method or instrumentation may not even exist for measurement of a particular physical property of samples with atomic-sized dimensions.
Point contacts can be made using a variety of methods. These include many variations of the so-called mechanical controlled break junction (“MCBJ”) method and scanning tunneling microscopy (“STM”), both of which utilize piezoelectric actuators (“piezos”) to close a gap between two opposite surfaces (herein referred to as a “tip” and a “substrate”). Other methods include the use of piezos, stepper motors, screw assemblies, and/or other moving mechanical parts to form a point contact between the tip and the substrate. Point contacts may also be formed by electrodeposition between two electrodes or electropolishing a fine wire.
However, there are problems with the existing conventional approaches. MCBJ-type methods, in which no one-to-one relationship between the displacement of the piezo and the movement of the tip relative to the substrate, suffer from parasitic mechanical and/or electrical noise. This results in producing an unstable point contact wherein the size is difficult to control, requiring separate calibration when a new tip is used, and involving inexact equations for determining displacement. In addition, the use of intermediate materials to support the displacement between tip and substrate in MCBJ-type methods may suffer from time-dependent or time-independent elastic/plastic behavior inherent in all materials, which also varies with temperature, thereby altering the displacement in a way that is difficult to predict.
Moreover, noise in the signal driving the piezo actuator in MCBJ, STM, or other similar methods causes the piezo to cause small changes in its shape. Although the variations in shape of the piezo are small, at the atomic scale they are large enough to make a point contact unstable—varying uncontrollably in size. Another drawback is the possible existence of drift in the signal driving the piezo, which has a similar effect. For example, FIGS. 1A and 1B show a gold point contact made by a MCBJ-type method, which suffers from uncontrollable change in size due to parasitic mechanical vibrations resulting from inadequate isolation. In this example, an approximately 50-atom gold point contact was formed at time t=0 s (FIG. 1A). The magnified view of the conductance trace (insets to FIG. 1A) show the presence of parasitic mechanical vibrations that causes the contact size to vary uncontrollably. FIG. 1B shows the Fourier transform of the conductance trace, which reveals the presence of mechanical vibrations of different amplitude and frequency.
The use of stepper motors, screw assemblies, moving mechanical parts, or straightforward use of piezos leads to similar mechanical and/or electrical noise as described above resulting in a lack of control over the size of point contacts.
Electrodeposition or electropolishing techniques are useful but limited to a narrow set of applications over a small range of temperature.
Overall, achieving stable point contacts free from parasitic mechanical and electrical interference, for measurement of a broad range of physical properties under different perturbations is complicated, imprecise, and has been difficult to integrate in one versatile system.